The blood brain barrier and blood cerebrospinal fluid barriers prevent many compounds in the blood stream from entering the tissues and fluids of the brain and central nervous system (“CNS”). It is generally recognized that nature provides these barriers to ensure a toxin free environment for neurologic function. The blood brain barrier (“BBB”) is of great importance for the maintenance of a constant environment for optimal neurological function. Most metabolic substrates (i.e. sugars and amino acids) are hydrophilic, and traverse these barriers only by specific carrier-mediated transport systems. Other molecules traverse the barriers more freely.
Loss of blood-brain barrier (BBB) function is an etiologic component of many neurological diseases. An intact BBB may restrict the delivery of certain therapeutic substances to the brain. Thus, measuring BBB function can be important to diagnosing disease progression and monitoring time-dependent changes in BBB integrity when chemotherapic penetration may be enhanced. At present, invasive and expensive techniques such as contrast-enhanced magnetic resonance imaging, CT scan and lumbar puncture are utilized to clinically assess BBB integrity.
Both sites of cerebrospinal fluid (“CSF”) formation, the choroid plexus as well as blood brain barrier endothelial cells, allow negligible passage of protein. Macromolecules such as polypeptides/protein, can cross an endothelial cell barrier primarily in three ways: between the cells through cell-cell junctions (paracellular pathway), through the EC, via pores (fused vesicles), or transcellularly via shuttling specific vesicles and receptors. Electron microscopic evidence suggests that macromolecules are shuttled across the endothelial barrier via vesicles.
BBB assessment by imaging or cerebrospinal fluid sampling is based on direct or indirect determination of protein permeability across the BBB. CNS proteins are normally asymmetrically distributed, with generally much high concentration in plasma than in CSF. Thus, the appearance of plasma proteins in cerebrospinal fluid (“CSF”) is a hallmark of numerous CNS disorders with presumed or overt BBB disruption. Only a few proteins are synthesized exclusively by, or are present in higher concentrations in CSF or interstitial compartment compared to the blood.
Samples of CSF can be intra surgically taken from the ventricles or from the sub-arachnoid space in the brain. An obvious limitation of intrathecal detection of blood brain-barrier intactness resides in the fact that sampling of CSF is invasive, and that the sample itself may be contaminated by the procedure. In addition, it has been known for a long time that a gradient in protein content exists from the brain to the lumbar cord. In fact, the concentrations of protein in segments distal to the site of CSF reduction (ventricles) are known to be much higher. It appears that, at least in part, the increased protein in the lumbar compartment of the CSF is due to a combination of protein secreted by parenchymal cells plus a small amount of protein leakage across the blood brain-barrier.
Ongoing or incipient systemic dysfunction of the heart, pancreas, liver or kidney can often be detected with the help of biochemical markers, whose specificity and known kinetics permit diagnostic and prognostic evaluation. The complexity of CNS function and the multiple kinetic parameters involved in bio-distribution across the blood brain barrier and within the brain parenchyma imposes a considerable burden for the interpretation of putative biochemical markers present in serum or cerebrospinal fluid. While changes in the composition of cerebrospinal fluid are commonly used to diagnose a variety of neurological diseases, the invasiveness of the procedures involved greatly diminishes their usefulness.